Doppler effect also known as Doppler shift,
is the change in frequency of a wave for
an observer moving relative to the source of the wave.
E.g. the siren of a fast approaching train you heard
usually much higher than a fast departing train.
The Doppler effect equation is:
f = f0 * (v + vr)/ (v + vs)
v: the velocity of waves in the medium
vr: The velocity of the receiver relative to the medium, positive if the receiver is moving towards the source
vs: The velocity of the source relative to the medium, positive if the source is moving away from the receiver
fs: Emitted Frequency
f: Received Frequency
The most widely used Doppler effect application is radar, e.g. police radar and radar for air traffic control.